Vascular smooth muscle contraction is suppressed by feedback dilation mediated by the endothelium. In skeletal muscle arterioles, this feedback can be activated by Ca signals passing from smooth muscle through gap junctions to endothelial cells, which protrude through holes in the internal elastic lamina to make contact with vascular smooth muscle cells. Although hypothetically either Ca or inositol trisphosphate (IP) may provide the intercellular signal, it is generally thought that IP diffusion is responsible. We provide evidence that Ca entry through L-type voltage-dependent Ca channels (VDCCs) in vascular smooth muscle can pass to the endothelium through positions aligned with holes in the internal elastic lamina in amounts sufficient to activate endothelial cell Ca signaling. In endothelial cells in which IP receptors (IPRs) were blocked, VDCC-driven Ca events were transient and localized to the endothelium that protrudes through the internal elastic lamina to contact vascular smooth muscle cells. In endothelial cells in which IPRs were not blocked, VDCC-driven Ca events in endothelial cells were amplified to form propagating waves. These waves activated voltage-insensitive, intermediate-conductance, Ca-activated K (IK) channels, thereby providing feedback that effectively suppressed vasoconstriction and enabled cycles of constriction and dilation called vasomotion. Thus, agonists that stimulate vascular smooth muscle depolarization provide Ca to endothelial cells to activate a feedback circuit that protects tissue blood flow.

Endothelial dysfunction in small arteries is a ubiquitous, early feature of cardiovascular disease, including hypertension. Dysfunction reflects reduced bioavailability of endothelium-derived nitric oxide (NO) and depressed endothelium-dependent hyperpolarization that enhances vasoreactivity. We measured smooth muscle membrane potential and tension, smooth muscle calcium, and used real-time quantitative polymerase chain reaction in small arteries and isolated tubes of endothelium to investigate how dysfunction enhances vasoreactivity. Rat nonmyogenic mesenteric resistance arteries developed vasomotion to micromolar phenylephrine (α 1 -adrenoceptor agonist); symmetrical vasoconstrictor oscillations mediated by L-type voltage-gated Ca 2+ channels (VGCCs). Inhibiting NO synthesis abolished vasomotion so nanomolar phenylephrine now stimulated rapid, transient depolarizing spikes in the smooth muscle associated with chaotic vasomotion/vasospasm. Endothelium-dependent hyperpolarization block also enabled phenylephrine-vasospasm but without spikes or chaotic vasomotion. Depolarizing spikes were Ca 2+ -based and abolished by either T-type or L-type VGCCs blockers with depressed vasoconstriction. Removing NO also enabled transient spikes/vasoconstriction to Bay K-8644 (L-type VGCC activator). However, these were abolished by the L-type VGCC blocker nifedipine but not T-type VGCC block. Phenylephrine also initiated T-type VGCC-transient spikes and enhanced vasoconstriction after NO loss in nonmyogenic arteries from spontaneously hypertensive rats. In contrast to mesenteric arteries, myogenic coronary arteries displayed transient spikes and further vasoconstriction spontaneously on loss of NO. T-type VGCC block abolished these spikes and additional vasoconstriction but not myogenic tone. Therefore, in myogenic and nonmyogenic small arteries, reduced NO bioavailability engages T-type VGCCs, triggering transient depolarizing spikes in normally quiescent vascular smooth muscle to cause vasospasm. T-type block may offer a means to suppress vasospasm without inhibiting myogenic tone mediated by L-type VGCCs.

Background: Background: While it has long been recognized that smooth muscle Na/K ATPase (NKA) modulates vascular tone and blood pressure (BP), the role of its accessory protein phopholemman (PLM) has not been characterized. The aim of this study was to test the hypothesis that PLM phosphorylation regulates vascular tone in vitro and this mechanism plays an important role in modulation of vascular function and BP in experimental models in vivo and in man. Methods: Mouse studies : PLM knock-in mice (PLM 3SA ), in which PLM is rendered unphosphorylatable, were used to assess the role of PLM phosphorylation in vitro in aortic and mesenteric vessels using wire myography and membrane potential measurements. In vivo BP and regional blood flow were assessed using Doppler flow and telemetry in young (14-16 weeks) and old (57-60 weeks) wild-type (WT) and transgenic mice. Human studies : We searched human genomic databases for mutations in PLM in the region of the phosphorylation sites and performed analyses within two human data cohorts (UK Biobank and GoDARTS) to assess the impact of an identified SNP on BP. This SNP was expressed in HEK cells and its effect on PLM phosphorylation determined using Western Blotting. Results: PLM phosphorylation at Ser63 and Ser68 limited vascular constriction in response to phenylephrine. This effect was blocked by ouabain. Prevention of PLM phosphorylation in the PLM 3SA mouse profoundly enhanced vascular responses to PE both in vitro and in vivo . In ageing WT mice PLM was hypophosphorylated and this correlated with the development of ageing-induced essential hypertension. In man we identified a non-synonymous coding variant, single nucleotide polymorphism rs61753924, which causes the substitution R70C in PLM. In HEK cells the R70C mutation prevented PLM phosphorylation at Ser68. This variant's rare allele is significantly associated with increased BP in middle-aged men. Conclusions: These studies demonstrate the importance of PLM phosphorylation in the regulation of vascular tone and BP and suggest a novel mechanism, and therapeutic target, for ageing-induced essential hypertension in man.